Telnyx (and maybe others) allow you to send them a Geolocation header in the SIP Invite to dynamically tell the carrier which 911 address to use. Telnyx provides a unique ID to use as the value of that header.
This would be used instead of having to use a separate DID for every extension.
Where, in FreePBX, would I actually find that Geolocation field to set a value in it per extension?
This is not the same. You can do both a static geolocation, meaning you submit the location and it has an identifier like “Ext200FL1RM200” and you can do realtime/dynamic which requires all the location details to be sent in content body. The Asterisk geolocation is the latter.